1. Field of the Invention
Embodiments of the present invention relate to electrochemical apparatuses including solid oxide fuel cells (SOFCs) and methods for making and using same.
More particularly, embodiments of the present invention relate to electrochemical apparatuses including SOFCs, where the SOFCs include thin film solid oxide fuel cells (TFSOFCs) including a porous metallic anode having a nano-porous surface structure and a micro-porous internal structure enabling deposition of a dense, impermeable, thin film, electrolyte layer on the porous anode surface, while maintaining anodic function. The present invention also relates to methods for making and using same.
2. Description of the Related Art
Fuel cells are energy-conversion devices that use an oxidizer (e.g., oxygen or air) to convert chemical energy in a fuel (e.g., hydrogen or low molecular weight hydrocarbons) into electrical energy. A solid oxide fuel cell (SOFC) generally comprises a solid electrolyte layer with an oxidizer electrode (cathode) on one side of the electrolyte and a fuel electrode (anode) on the other side. The electrodes are required to be porous, or at least permeable to the oxidizer at the cathode and the fuel at the anode, while the electrolyte layer is required to be dense so as to prevent leakage of gas across the layer. A thin film solid-oxide fuel cell (TFSOFC) has a thin electrolyte layer, on the order of 0.1 micrometers or microns (μ or μm) to 5 μm thick, as described, for example, in U.S. Pat. No. 6,645,656. The use of a thin electrolyte layer reduces the operating temperature significantly. A significant reduction in operating temperature increases the reliability of the fuel cell, and allows wider choices of materials for TFSOFC applications. Using the TFSOFC design may also reduce materials costs and reduce the volume and mass of the fuel cell for a given power output.
U.S. Pat. No. 6,645,656 discloses physical and chemical deposition techniques to synthesize basic components of a TFSOFC consisting of an electrolyte thin film, e.g., yttria stabilized zirconia (although a number of other oxide electrolytes could be utilized), a thin film cathode layer, e.g., lanthanum, strontium cobalt oxide (although a number of other oxide cathodes could be utilized) both deposited on a porous metal anode, e.g., nickel (although a number of other metal anodes could be utilized).
Thin film oxide deposition technologies such as pulsed laser deposition (PLD) or metal organic chemical vapor deposition (MOCVD) can be used for the deposition of the oxide electrolyte as well as for the conducting oxide cathode layer. PLD is an ideal vehicle to develop very thin films for TFSOFC applications, while MOCVD is good for large area thin film fabrication. Sputtering, evaporation, sol-gel, metal organic deposition (MOD), electron beam evaporation, chemical vapor deposition (CVD), molecular beam epitaxy (MBE), or other oxide film deposition techniques can also be used. For an effective thin film electrolyte layer having a thickness between 0.1 μm and 5 μm to be deposited onto the porous metal anode, it must be dense and impermeable to gas flow. Hence the deposition of the thin electrolyte layer onto the anode requires that the surface of the porous metal anode have a pore size that is equal to or less than the thickness of the electrolyte layer, thus allowing for full coverage and closure of the pores. The most common porous metallic-based anode materials for solid oxide fuel cells are nickel (Ni)-cermets prepared by high-temperature calcination of NiO and yttria-stabilized zirconia (YSZ) powders as described in U.S. Pat. Nos. 6,589,680 B1; 7,842,200 B2; in Suzuki et al, “Impact of Anode Microstructure on Solid Oxide Fuel Cells”, Science, 325, pp. 852(2009); in Chen et al, “Hierarchically Oriented macroporous anode-supported solid oxide fuel cell with thin ceria electrolyte film”, Applied Materials & Interfaces, 6, lop. 5130(2014); in Y. Liu and M. Liu, “Porous SOFC anode prepared by sublimation of an ikmiscible metal oxide during sintering”, Electrochem and Solid State Letters, 9, pp. B25 (2006). High-temperature calcination is essential in order to reduce the nickel oxide to metallic nickel and thus form the necessary electronic conductivity and porosity in the anode structure.
While several anode structures and methods for making the anode structures are known, there is still a need in the art for anode structures and methods for manufacturing same, where the porous metallic anode with nano-porous surface structure and micro-porous internal structure enables the deposition of a dense, impermeable thin film electrolyte layer on the porous anode, while maintaining anodic function